Cathode structure for high current, low pressure discharge devices

ABSTRACT

An improved cathode structure for high current, low pressure electric discharge devices is described as comprising an oxide emissive mix coated filament mounted axially within a hollow open-ended refractory metal cylinder. Unusually high discharge currents on the order of 2 to 50 amperes are provided with this electrode structure for long periods of time.

United States Patent Johnson et al.

CATHODE STRUCTURE FOR HIGH CURRENT, LOW PRESSURE DISCHARGE DEVICES lnventors: Peter D. Johnson, Schenectady;

John M. Anderson, Scotia, both of NY.

Assignee: General Electric Company,

Schenectady, NY.

Filed: Mar. 4, 1974 Appl. No.: 448,018

U.S. Cl. 313/212; 313/217; 313/218;

313/356 Int. Cl. H0lj 61/08 Field of Search 313/212, 217, 218, 356,

References Cited UNITED STATES PATENTS 10/1954 Arnott 313/212 X 51 May 13, 1975 2,935,637 5/1960 Cortese 313/492 X 3,121,184 2/1964 Fox 313/492 X 3,215,882 11/1965 Toomcy 313/217 X 3,657,591 4/1972 Johnson 313/227 X Primary Examiner-Siegfried H. Grimm Attorney, Agent, or Firm-Jerome C. Squillaro; Joseph T, Cohen; Julius J. Zaskalicky [57] ABSTRACT An improved cathode structure for high current, low pressure electric discharge devices is described as comprising an oxide emissive mix coated filament mounted axially within a hollow open-ended refractory metal cylinder. Unusually high discharge currents on the order of 2 to 50 amperes are provided with this electrode structure for long periods of time.

11 Claims, 2 Drawing Figures PATENIED MAY 1 3197s CATHODE STRUCTURE FOR HIGH CURRENT, LOW PRESSURE DISCHARGE DEVICES The present invention relates to electric discharge devices, and more particularly to improved cathode structures for high current, low pressure discharge devices useful for enhanced photochemical stimulation, high intensity fluorescent lamps and high output gas lasers.

High current, low pressure discharges in mercury and other metal vapors and inert gases, for example, have been found to be efficient and intense sources of ultraviolet radiation useful in photochemistry and numerous other applications. The requirement of high discharge currents (2 to 50 amperes or more) at low gas pressure strains or exceeds the capabilities of conventional electrodes. Ordinary fluorescent lamp electrodes, for example, are capable of delivering currents of up to 3 or 4 amperes for short periods of time, such as those required for experimental work. However, two or more filaments are generally employed at each electrode where such current densities are required. These filaments, however, are not adequate for higher currents and longer life required for recently developed practi cal applications. Further, a plurality of filaments occupy so much space that the electrode section of the discharge tube must generally be of larger diameter than the section occupied by the positive column of gases. Still further, other externally heated cathodes such as the barium aluminate dispenser type cathode or the massive tungsten-thorium filaments used for high current gas laser applications, while providing the de sired arc current, exhibit a reduced overall efficiency because they require on the order of 50 watts or more to maintain the temperature of the filament at a useful value.

Electric lamps emitting ultraviolet radiation, for example, generally utilize a gaseous discharge including mercury as the emitting species. In most prior art devices utilized for this purpose, the lamp parameters for low current density, generally less than 0.2 ampere/cm", and low pressure of emitting species, generally below one Torr, have an emission at wavelengths above 2300 A.U., primarily that of the mercury 2537 A.U. line which is so strong as to dominate such ultraviolet emission. The use of such electric lamps for photochemical reactions, such as cross-linking of polymers and breaking of polymeric bonds, is very inefficient. Further, other ultraviolet emitting lamps when operated at high pressures and high currents still only emit useful ultraviolet radiation at wavelengths longer than 2300 A.U..

One of the primary difficulties with ultraviolet emitting lamps operating at high currents is the blackening of the discharge tube walls caused by sputtering or evaporation of the emission mix or metal from the electrode structure. As a result of this blackening, the discharge tube becomes opaque to ultraviolet radiation and hence after a short period of time the lamp ceases to produce useful ultraviolet radiation.

It is, therefore, an object of this invention to provide an improved electrode structure for an electric discharge device which overcomes the aforementioned disadvantages and exhibits a low voltage drop thereby providing low power loss at the electrode.

It is a still further object of this invention to provide a high current, low pressure discharge lamp including an electrode structure which provides improved lamp efficiency.

It is still a further object of this invention to provide an efficient ultraviolet emitting lamp having a high current, low pressure discharge with an improved electrode structure.

Briefly stated in accord with the present invention, we provide an improved cathode structure for high current, low pressure discharge devices in which the cathode structure comprises an oxide emissive mix coated on a filament mounted axially inside a hollow cylinder of refractory metal. When included in an evacuable envelope containing a quantity of mercury sufficient to yield, under operating wall temperatures of 25 to C, an optimum pressure of mercury vapor of approximately 2 X 10' to 0.1 Torr, the emission of high intensity far ultraviolet radiation, largely at a wavelength of 1942 A.U. is achieved. Under these conditions, current densities of the order of 2 to 50 amperes/cm are achieved with the ultraviolet output greatly improved at higher current densities.

The novel features believed characteristic of the present invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood with reference'to the following detailed description taken in conjunction with the appended drawings in which:

FIG. 1 is a horizontal view, with parts broken away of a lamp constructed in accord with the present invention and suitable for operation for the production of high intensity ultraviolet radiation; and

FIG. 2 is a partial perspective view of one embodiment of an improved electrode structure in accord with the present invention.

FIG. 1 illustrates, by way of example of one embodiment of our invention, a far ultraviolet lamp including an evacuable envelope, represented generally at 10. The evacuable envelope 10 is preferably formed of an ultraviolet transmissive material, such as fused quartz, for example, so that ultraviolet radiation freely passes through the envelope. The evacuable envelope 10 includes end members 11 and 12, respectively, which each support an electrode assembly 13. The electrode assemblies include lead-in and support members 14 and 15 substantially parallel to each other and extending through the end members 11 and 12, respectively. The end members 11 and 12 provide a graded seal at the ends of the evacuable envelope 10.

The desired current and voltage to operate the lamp are provided by a power supply means, capable of supplying the requirements of the lamp in operation and may take a number of forms. For example, FIG. 1 illustrates generally a current regulating transformer 16 including a primary winding 17 and a secondary winding 18. The primary winding 17 is connected to a suitable source of current and voltage and the secondary winding 18 is connected to electrodes 15 at each end of the evacuable envelope 10.

In accord with one embodiment of our invention, FIG. 2 illustrates the electrode assembly 13 particularly useful as a cathode as comprising an open-ended cylinder 21 and an oxide emissive coated filament 23. As will be described more fully below, under alternating current excitation, the entire electrode assembly 13 functions alternately as a cathode and then an anode. The cylinder 21 is preferably formed of a refractory metal material, such as molybdenum, tantalum, tungsten, rhenium, or any other metal having negligible vapor pressures at temperatures of approximately l,200 to l,300C. By way of example, the refractory metal cylinder 21 may have a wall thickness of between 1 and .mils, a diameter of approximately 0.5 to 2.0 centimeters, and an axial dimension approximately 2 or 3 times the diameter.

FIG. 2 also illustrates the lead-in and support member extending into the cylinder 21 and attached to the inner surface of the cylinder wall by suitable means, such as spot welds. The lead-in and support member 15 has at one end thereof a support rod 22 extending orthogonally from the support member 15 for supporting one end of the filament 23, illustrated in the form of a helix. The other end of the filament is supported by lead-in and support member 14. As illustrated, the helical filament 23 is mounted coaxially in the cylinder 21 and includes a coating 24 of an emissive mix such as those used in high output fluorescent lamps. These emissive mixes generally include one or more of the alkaline earth oxides or one or more of the rare earth oxides, or one or more of the oxides of thorium, yttrium, zirconium, hafnium, or tantalum. The filament material itself is also not critical and may include materials such as tungsten, tantalum or rhenium, for example.

Prior to operation of the lamp, it is necessary to coat the inner wall of the cylinder 21 with emissive mix. This may be achieved, for example, by providing a sufficient current flow through the emissive coated filament 23 to achieve a higher-than-normal temperature, e.g., about l,l00C, for the alkaline earth emissive mix referred to above. As a result of this heating, the emissive mix diffuses to the inner wall of the cylinder 21 and provides a coating thereon. After the coating is applied, the lead-in and support members 14 and 15 are electrically connected together, as illustrated in FIG. 1.

The lamp is also charged with a low pressure of an inert gas such as krypton, argon or neon and a sufficient quantity of mercury 25. The quantity of vaporizable mercury present as the charge 25 is sufficient to produce a mercury vapor pressure of 2 X 10' to 0.1 Torr at operating wall temperatures of 25 to 80C. The inert gas is at a low pressure, as for example 0.5 25 and preferably approximately 2 5 Torr of krypton. As pointed out in US. Pat. No. 3,657,591, the excitation of the emitting mercury is greatly enhanced by interactions with a partial pressure of krypton gas. The enhanced excitation results in greatly enhanced emission intensity.

operationally, the lamp is started by the application of a line voltage, which may be of any desired voltage, but may conveniently be 120 or 240 volts, to the primary of the excitation transformer 16. The initial application of the voltage causes excitation of the krypton, which immediately ionizes the mercury atoms initiating the discharge. The establishment of a sufficient pressure of mercury permits ionization thereof and causes transfer of the discharge to mercury, as the conducting specie. At these low pressures and high current densities, an efficient emission of photochemically useful radiation less than 2,000 A.U. and specifically at 1849 A.U. and 1942 A.U. radiation is obtained.

As pointed out in the above-identified patent, in order to optimize the 1942 A.U. emission from the lamps, the bulb wall temperature (i.e., the minimum temperature of the interior of the bulb wall exists during steady-state operation) is maintained within the range of approximately 15 to 100C, and preferably within a range of approximately 25 to C, for optimum emission of 1942 A.U. resonance mercury ion radiation. This preferred temperature yields an operating pressure of mercury within the envelope within the range of approximately 2 X 10 Torr to 0.1 Torr.

The current density within the lamp is also maintained in the desired range by appropriately adjusting the total current through the discharge and the diameter of the narrow, U.V. transmissive portion of the lamp envelope. The total current is controlled by external impedances, and is adjusted to obtain maximum output from the radiating spectroscopic states of the radiant species.

In general, for alternating current operation, lamps having the novel electrode structure in accord with our invention may readily operate at voltages from 20 to 200 volts A.C. at current densities of between approximately 2 and 50 amperes/cm and within a preferred range of between approximately 5 and 25 amperes/cm A typical lamp configuration for the attainment of such operation, namely at a current density of approximately 10 amperes/cm and a pressure of approximately 3 X 10 Torr, may readily be obtained within a lamp envelope having an interior diameter within the ultraviolet transmissive region of approximately 14 mm and a length of approximately 15 cm between the cathode structures.

Lamps operated with the electrode structure in accord with the present invention exhibit numerous ad- -vantages over prior art electroded lamps. For example,

lamps constructed with our improved electrode structure have unusually high intensity of emission of the 1942 A.U. line. Another advantage of lamps constructed with electrode structures in accord with the present invention is that at current densities above approximately 3 amperes/cm the discharge occupies the cylindrical electrode member 21 in a completely diffuse (hollow cathode) mode providing high current output and low electrode damage. This is believed to be attributable to the emissive mix evaporated onto the inside of the cylinder during activation which permits the entire electrode structure to function as a cathode, Still another advantage of our invention is an additional function provided by the cylinder 21; namely, the cylinder intercepts any subsequently evaporated or sputtered emissive mix thereby preventing darkening of the walls of the envelope. For example, a lamp similar to that illustrated in FIG. 1 operated at 10 amperes discharge current exhibited negligible darkening of the envelope with no discernible loss of UV output after 500 hours of operation.

In accord with another feature of the present invention, the cathode fall (voltage drop) when operating in the hollow cathode mode is only a few volts, i.e., under 4 volts. Conventional electrodes, on the other hand, exhibit voltage drops in excess of 7 volts. Accordingly, discharge lamps having electrodes constructed in accord with our invention have reduced power loss at the electrodes which results in lower cathode temperatures, i.e., on the order of 1000C at a current of IO amperes, for example. Further. since the electrodes in accord with our invention are operated in a hollow cathode mode, a more diffuse discharge from a much larger effective surface area (substantially the entire electrode structure) is provided, thereby minimizing electrode deterioration and discharge tube wall darkening.

By the foregoing we have described a new and improved electrode structure which provides a diffuse discharge from a large surface area, resulting in lower cathode temperatures, longer cathode life and reduced cathode fall compared to conventional filamentary cathodes. When employed in a far ultraviolet emitting lamp at current densities in the range of 2 to 50 amperes/cm these lamps emit radiation principally at a wavelength of 1942 A.U.

While the invention has been described with reference to certain specific examples or preferred embodiments thereof, many modifications and changes will readily occur to those skilled in the art. For example,

i in addition to the use of high current, low pressure discharges for photochemical applications, the invention can also be used in other applications such as very high intensity fluorescent lamps and high output gas lasers. Also, those skilled in the art can appreciate that in addition to alternating current operation, the novel electrode structure may be operated on direct current, if desired. Additionally, other chemically inert gases besides krypton may be used, if desired. Accordingly, the appended claims are intended to cover all such modifications, changes and uses as fall within the sphere and scope of the foregoing disclosure.

What we claim as new and desire to secure by letters patent of United States is:

1. In a high current, low pressure discharge device including anevacuable envelope having a partial pressure of an ionizable material therein and electrode structures for establishing an electric discharge therein, said electrode structures including an improved cathode structure comprising:

a refractory metal cylinder;

an emissive mix coated filament coaxially supported within said cylinder;

means for electrically connecting the ends of said filament together; and means for applying a voltage to said electrode structures for establishing an electric discharge within said envelope.

2. The device of claim 1 wherein said evacuable envelope includes a partial pressure of a material selected from the group consisting of krypton, xenon and argon within the range of approximately 0.5 to 5 Torr and a quantity of mercury sufficient under lamp operating conditions to maintain a partial pressure of mercury of approximately 2 X 10 to 0.] Torr.

3. The device of claim 2 wherein the operating conditions include maintaining the coldest portion of the envelope at a temperature of approximately 25 to C.

4. The device of claim 1 wherein the inner wall of said cylinder includes an emissive mix coating thereon.

5. The device of claim 2 wherein said discharge comprises ionized mercury which when excited by current densities of approximately 2 to 50 amperes/cm emits ultraviolet radiation at wavelengths shorter than 2000 A.U.

6. The device of claim 5 wherein said radiation is principally at 1942 A.U.

7. The device of claim 1 wherein said refractory metal is selected from the group consisting of molybdenum, tantalum, tungsten, and rhenium.

8. The device of claim 1 wherein said evacuable envelope comprises an ultraviolet transmissive material.

9. The device of claim 1 wherein said cylinder substantially prevents evaporated or sputtered emissive mix emitted by said filament from depositing on the walls of said evacuable envelope.

10. The device of claim 5 wherein said current density is greater than 3 amp/cm and said electric discharge occupies said metal cylinder in a completely diffuse mode providing high current output with minimum cathode damage.

11. The device of claim 6 wherein the voltage drop of said cathode structure is less than 4 volts. 

1. In a high current, low pressure discharge device including an evacuable envelope having a partial pressure of an ionizable material therein and electrode structures for establishing an electric discharge therein, said electrode structures including an improved cathode structure comprising: a refractory metal cylinder; an emissive mix coated filament coaxially supported within said cylinder; means for electrically connecting the ends of said filament together; and means for applying a voltage to said electrode structures for establishing an electric discharge within said envelope.
 2. The device of claim 1 wherein said evacuable envelope includes a partial pressure of a material selected from the group consisting of krypton, xenon and argon within the range of approximately 0.5 to 5 Torr and a quantity of mercury sufficient under lamp operating conditions to maintain a partial pressure of mercury of approximately 2 X 10 3 to 0.1 Torr.
 3. The device of claim 2 wherein the operating conditions include maintaining the coldest portion of the envelope at a temperature of approximately 25* to 80*C.
 4. The device of claim 1 wherein the inner wall of said cylinder includes an emissive mix coating thereon.
 5. The device of claim 2 wherein said discharge comprises ionized mercury which when excited by current densities of approximately 2 to 50 amperes/cm2 emits ultraviolet radiation at wavelengths shorter than 2000 A.U.
 6. The device of claim 5 wherein said radiation is principally at 1942 A.U.
 7. The device of claim 1 wherein said refractory metal is selected from the group consisting of molybdenum, tantalum, tungsten, and rhenium.
 8. The device of claim 1 wherein said evacuable envelope comprises an ultraviolet transmissive material.
 9. The device of claim 1 wherein said cylinder substantially prevents evaporated or sputtered emissive mix emitted by said filament from depositing on the walls of said evacuable envelope.
 10. The device of claim 5 wherein said current density is greater than 3 amp/cm2 and said electric discharge occupies said metal cylinder in a completely diffuse mode providing high current output with minimum cathode damage.
 11. The device of claim 6 wherein the voltage drop of said cathode structure is less than 4 volts. 